This invention relates to filtration of fluids employed in completion and treatment of oil and gas wells. After a well is drilled, the well bore wall, and most critically, the wall at the location in which the well bore intersects the producing oil and/or gas formation, has become contaminated with the drilling fluid or "mud" utilized in the drilling operation. In order to remove the mud solids, commonly termed filter cake, from the well bore wall and to clean the formation pores, a number of different fluids may be pumped into the well and circulated. In some formations, acid, such as hydrochloric acid, acetic acid, formic acid, hydroflouric acid and others, with appropriate retarders, inhibitors, surfactants and other additives, all of which are well known in the art, are used to treat the formation by removing filter cake, shrink or remove clays, alleviate cement damage to the formation and remove iron deposits, scale deposits and other contaminants present in the well bore and formation. In other formations, non-acidic treatments may similarly be employed. All of the above treatments may in and of themselves produce the desired beneficial results in the producing formation, or may in many instances serve as pre-treatments for subsequent operations such as fracturing or gravel packing.
In any event, a major problem experienced by service companies and operators is the introduction into wells of contaminants suspended in the treatment fluids. Such contaminants include rust from tanks, lines and pumps, sediment from tank bottoms due to incomplete cleaning after prior jobs, bacterial contaminants, and the dust and dirt generally present at a well site. Ideally, treatment fluids would be filtered immediately prior to entry into the well.
Numerous prior art attempts have been made to provide effective, single-pass filtering for some treatment fluids, as discussed in U.S. Pat. No. 4,428,425, wherein plate-type filters are employed to filter completion brines. Such filters are effective, but are restricted to very low pressures (several hundred psi maximum) and so must be employed upstream of high pressure pumps, rather than on the discharge side thereof immediately prior to injection in the well. Moreover, such filters cannot, by their design, be used to filter acids. Another approach has been the use of cartridge-type filters in banks, wherein one bank of filters can be shut off and filter cartridges changed while filtering continues through the other bank. Such a device, in the form of a transportable skid unit, has been developed and is sold by Special Projects Manufacturing, Inc. of Forth Worth, Tx., as the SPM High pressure Filter Unit. This apparatus, however, possesses inherent design defects as it uses welded joints and pipe thread connections throughout, both of which are extremely susceptible to failure after repeated or lengthy exposure to acids, and the latter of which tends, particularly under high pressures, to leak badly.